The present invention relates to a high efficiency semiconductor light emitting diode (LED) of the edge-emitting type with a cutoff waveguide for use in high speed fiber optical communications.
The two types of semiconductor LED's most often used in fiber optical communication systems are the surface etched well emitter (Burrus type) and the edge-emitter. In the surface emitter, light is emitted from the surface of a planar LED structure along a path that is perpendicular to the p-n junction plane of the device. An optical fiber is aligned with this path to receive the emitted light. A well is etched into the top of the structure to enable the fiber to be as close as possible to the light emitting active region of the device.
The structure of the edge-emitter is very similar to that of a double heterojunction semiconductor laser. In the edge-emitter, the device is composed of layers of semiconductor material doped so that a p-n junction plane is formed. When the junction is forward biased, injected carriers (electrons and holes) recombine in the active layer and light is generated. The light is emitted from an edge of the device along a path which is parallel to the junction plane of the active layer. An optical fiber is aligned with this path at the edge of the device where the light is emitted.
Edge-emitters are potentially cheaper to fabricate and easier to package than surface emitters; however, the total optical power output of an edge-emitter is typically a fraction of that from comparable surface emitters. This lower output power level is caused by the reabsorption of light within the active region of the device.
In conventional edge-emitting LED's, the active layer is typically surrounded by two confining layers which in turn are surrounded by two optical guide layers and light is emitted from the LED after multiple internal reflections at the interface between a confining layer and an optical guide layer. Because the material composing the active layer absorbs optical energy produced when the carriers recombine, the active layer is usually constructed so that it is relatively thin (about 0.1 microns) in order to increase the optical efficiency of the device. However, manufacturing LED's with such thin active layers is difficult and therefore expensive.